The present invention relates to a method for treating a surface of a shaped article made of a polypropylene resin, such as a bumper for automobiles, and more particularly to a method for cleaning a surface of a shaped article made of a polypropylene resin with a detergent having a high detergency and an enhanced safety before a coating process.
A polypropylene resin has been used in a wide variety of fields due to its low weight, excellent mechanical properties, high moldability, etc. In recent years, such a polypropylene resin is also widely used as shaped articles such as bumpers for automobiles, etc., due to its excellent reusability. The bumper made of such a polypropylene resin is usually coated with a coating composition having the same color as that of a steel body of the automobile.
In this case, however, there has been a tendency that various contaminants such as a mold release agent, a mold rust-preventing agent, an operating oil (machine oil), a conveyor oil, oily dirt from human hands, etc. are attached to a surface of the bumper during its manufacturing processes. This leads to such problems that a coating composition suffers from crawling on the contaminated surface of the bumper, and that the resulting coating tends to peel off therefrom.
Under these circumstances, the bumper has been usually subjected to a pretreatment to remove the contaminants from the surface thereof by using an organic solvent. A typical organic solvent used for such a purpose includes trichloroethane. Although trichloroethane is capable of dissolving oily contaminants and therefore exhibits a high detergency, it causes air pollution problems such as destroying an ozone layer and a water pollution. On these grounds, it is expected that the use of trichloroethane is legally banned in the near future. Other organic solvents used include aromatic compounds such as toluene, kerosene, gasoline, petroleum benzine, isopropyl alcohol, etc. However, these petroleum-type solvents or alcohol-type solvents disadvantageously show a high volatility and a high flammability.
As a result, there has been recently proposed a method in which an aqueous detergent such as a surface-active agent, an acidic degreasing and deterging agent, etc., which does not suffer from the above-mentioned problems, is used to remove oily contaminants from the surface of the bumper. However, conventional aqueous detergents do not have sufficient effects on a wide range of contaminants including various organic or inorganic substances.
There has been an increased demand to develop methods in which an aqueous detergent with high detergency for various contaminants is used without suffering from the above-mentioned problems. For example, Japanese Patent Laid-Open No. 2-284681 discloses a degreasing and deterging method in which coarse alkali salts and a white kerosene are used in a degreasing and deterging agent. This degreasing and deterging method still causes the above-mentioned problems because the degreasing and deterging agent contains white kerosene.
Japanese Patent Laid-open No. 2-55799 discloses a detergent which comprises an alkali primarily in the form of a potassium salt or an acid primarily in the form of a phosphoric acid or a phosphoric acid salt, polyethylene glycol, a nonionic surface-active agent (HLB: 12-18) and a defoaming agent. This detergent is likely to generate a nutrition-rich effluent because phosphorus and nitrogen are contained therein. Though the detergent contains a sodium salt or a potassium salt of phosphoric acid as an alkali builder, its detergency is poor because this alkali builder has considerably high saponification degree and dispersibility. Though the detergent also contains a phosphoric compound as an acid builder, a sufficient detergency is not obtained as well, since such an acid builder has poor saponification.
WO 9118046 discloses a two-step process comprising a cleaning step with an alkali and a cleaning step with an acid. However, this process requires a long period of time,-resulting in increased cost.